The present disclosure relates to an image forming apparatus employing a photosensitive drum, and more particularly to a method of removing moisture from the surface of a photosensitive drum.
In image forming apparatuses, such as copiers, printers, and facsimile machines, that rely on electrophotography, developer (hereinafter referred to also as toner) in powder form is typically used. According to a commonly adopted process, an electrostatic latent image formed on an image carrying body such as a photosensitive drum is made visible with toner stored in a developing device, and the toner image is then transferred onto a recording medium and is then fused. The photosensitive drum has a photosensitive layer with a thickness of ten micrometers to several tens of micrometers formed on the surface of a cylindrical base member. Depending on the main material of the photosensitive layer, photosensitive drums are classified into organic photosensitive drums, selenium-phosphorous photosensitive drums, amorphous silicon (hereinafter abbreviated to a-Si) photosensitive drums, etc.
Organic photosensitive drums are comparatively inexpensive, but wear easily and require frequent replacement. Selenium-phosphorous photosensitive drums have longer lifetimes than organic ones, but are difficult to handle because of the toxicity of their material. In contrast, a-Si photosensitive drums are expensive as compared with organic ones, but are easy to handle because of the non-toxicity of their material, have high hardness and excellent durability (five times as high as organic ones), and exhibit almost no deterioration in their properties as photosensitive drums even after a long period of use, maintaining high image quality. These thus serve as excellent image carrying bodies with low running cost and high safety to the environment.
It is known that image forming apparatuses employing such photosensitive drums are, due to their properties, depending on the conditions of use, prone to so-called image dropouts, which make the formed image scratchy, or blurred at the edge. Image dropouts occur by the following mechanism. When the surface of a photosensitive drum is electrically charged by a charging device, the electric discharge by the charging device generates ozone. The ozone decomposes components of the atmosphere, generating ionic products such as NOx and SOx. These ionic products are water-soluble, and thus adhere to the photosensitive drum and bite into the coarse structure with an asperity of about 0.1 μm at the surface of the photosensitive drum. They thus cannot be removed by a cleaning system employed in general-purpose products, and they even absorb moisture in the atmosphere and reduce the resistance at the surface of the photosensitive drum. This causes a lateral flow of potential at the edge of the electrostatic latent image formed on the surface of the photosensitive drum, possibly resulting in image dropouts. This phenomenon is particularly notable with a-Si photosensitive drums, which are less prone to surface wear against a blade or the like and which have at the surface a molecular structure that easily absorbs moisture.
There have conventionally been proposed various methods for preventing occurrence of such image dropouts. For example, according to one known method, a heat generating body (heater) is provided inside a photosensitive drum, or inside a sliding member that is in contact with a photosensitive drum, and heat is generated by controlling the heat generating body according to temperature and humidity detected by a temperature/humidity sensor inside the apparatus so as to evaporate moisture that has attached to the surface of the photosensitive drum, thereby to prevent occurrence of image dropouts.
However, the method relying on a heater arranged inside a photosensitive drum requires use of sliding electrodes for connection between the heater and a power supply. Due to the presence of sliding parts for connection between the heater and the power supply, inconveniently, as the total rotation period of the photosensitive drum increases, the sliding parts become prone to bad contacts. Moreover, in the current trend for energy saving and environment-friendliness, there is a strong demand for reduced electric power consumption during standing-by and during ordinary printing. In particular, in apparatuses having a plurality of drum units, such as tandem-type full-color image forming apparatuses, their electric power consumption is high, and therefore it is not desirable to incorporate a heater. Heat around a cassette heater or a fusing device may be transferred to around a photosensitive drum, but this is ineffective because a component nearby, such as a developing device, is also heated.
Thus, there is known an image forming apparatus in which a weak charge period in which a charge voltage containing a DC voltage alone or a charge voltage having an AC voltage lower than during image formation is superimposed on a DC voltage is provided in a predetermined period before the start of, or after the end of, a regular charge period or between a plurality of regular charge periods so as to suppress generation of electric discharge products resulting from application of a charge bias other than during image formation.
There is also known an image forming apparatus that can execute a moisture removal mode in which are sequentially performed a first moisture removal process in which moisture is removed from the surface of a photosensitive drum with a cleaning blade, a second moisture removal process in which toner on a developing roller is transported to the photosensitive drum and moisture on the surface of the photosensitive drum is absorbed in the toner and is removed together with the toner, and a third moisture removal process in which a voltage is applied to a charging roller to remove moisture on the surface of the charging roller and the photosensitive drum.